As modem aircraft are designed to take advantage of the cost, weight, and performance advantages afforded by DC power generation and distribution as the primary power for the aircraft utilization equipment, the requirements governing power quality have become much more stringent. Not only must the DC voltage regulation provide a set voltage that is stable over the entire speed and load range of the generator and limit the current output of the generator to prevent system damage during faults, but it must also meet increased dynamic performance requirements. These performance requirements include rapid transient recovery and low source impedance characteristics in the range of frequencies governed by the voltage regulator.
One voltage regulation approach to meet the performance requirements of a modem DC power generation and distribution system is described in U.S. Pat. No. 4,933,622, issued Jun. 12, 1990, for a Circuit and Method for Discharging DC Filter Capacitors, to David A. Fox and Ralph Jessee, and assigned to the assignee of the instant application. This patent describes that the output filter capacitor of the DC generation system must be sufficiently large to provide a low output impedance, but that the use of such a large capacitor adversely effects the dynamic performance of the system due to its inherent energy storage properties and slow discharge rate, especially at light system loads. The Fox et al. system provides a capacitor discharge circuit which provides a discharge path for the capacitor voltage when it is seen to increase beyond a set point, particularly in response to a sudden load removal from the system. The sensing control for this circuit is derived primarily from a DC voltage sensor, although Fox et al. indicates that providing AC voltage sensing in addition to the DC is an option as well. While this system presents a marked improvement over the previous control systems known in the art, the additionally capacitive discharge circuitry may be disadvantageous in some applications.
Another method and circuit for voltage regulation of DC power sources is described in U.S. Pat. No. 4,807,106, issued Feb. 21, 1989, to Donal Baker, David Fox, and Roger Thornton, and assigned to the assignee of the instant application. This system does not require the capacitive discharge circuitry of the above referenced system, but instead utilizes an AC voltage sense summed with the generator exciter field current to control the DC output. The control additionally provides that the AC voltage based control be trimmed by a DC voltage sense. This DC trim loop typically has a limited range to allow compensation only for loading and temperature effects. The response time of this DC voltage trim loop is also delayed so as to not interfere with the AC loop to allow primary generator control to track the AC generator output. This is intended to ensure that the output of the generator does not collapse during no load conditions when the output capacitor is not being discharged by any connected load.
While both of these systems improve the DC output voltage regulation over the prior known regulation schemes, transient requirements for new aircraft are much more aggressive, especially in the required source impedance characteristic in the range of frequencies governed by the voltage regulator. The instant invention meets these latest functional requirements for voltage regulation of DC power generation systems without the need for added output capacitance and its associated capacitive discharge circuitry or actively clamped and time delayed DC trim loops.